1. Field of the Invention
The present invention relates to a cutting tool which is mounted on the spindle of a cutting machine, and to a cutting method by using the above tool.
2. Related Art
To perform cutting work using a machine tool or the like, a cutting tool, such as an end mill, having a cutting edge at its tip is mounted on a spindle, and the cutting tool is rotated and contacted with a workpiece to cut out a desired shape.
As an example of such art, Japanese Patent Laid-Open Publication No. 2001-9603 discloses a cutting method. The cutting method disclosed in that publication is especially applicable to the cutting of inner corners of a pocket hole or the like. In the disclosed inner corner cutting method, a cutting tool having a cross-sectional shape as shown in FIG. 9 and a plurality of cutting parts along the circumference at the bottom edge of the tool is mounted on the spindle of the machine tool, a rotation angle of the cutting tool and a position of the spindle on a coordinate system of the machine tool or the workpiece are synchronously controlled, and a relative position of the cutting tool to the workpiece is controlled according to the rotation angle of the cutting tool so that outer end points of the cutting parts draw a locus of movement along a desired inner corner shape, thereby moving while cutting in a direction of the tool's axis. FIG. 10 shows an example of the cutting edge locus of the outer end points of the cutting parts according to the above machining method and the relative movement locus of the cutting tool to the workpiece which is necessary to draw the cutting edge locus.
However, when an inner corner of a pocket hole or the like is cut as described in the above-cited publication (No. 2001-9603), it is necessary to perform accurate synchronous control of the rotating axis which controls the rotation angle of the cutting tool and a feed axis which controls relative movement (generally, movement of the X-, Y-axes on a plane perpendicular to the spindle axis) of the cutting tool. Because the feed rate of the feed axis is variable, both axes accelerate and decelerate during operation, and the axes have different following characteristics, synchronization is a problematic and complicated process. As a result, a path error ε commonly occurs at the outer end point of the cutting part of the cutting tool because of a loss of synchronism, resulting in a machined shape error such as shown in FIGS. 11A, 11B.
The synchronous control has a problem that when the spindle moves from a command block (a command for a rotation angle of the spindle, a command for a position of the spindle) to the spindle, on which the cutting tool is mounted, to the next command block in certain timing, the cutting edge of the cutting part of the cutting tool rotates, and a distance in a direction perpendicular to the moving direction of the spindle varies as shown in FIG. 12. The result is a path error ε on the circumference of the cutting part in the moving path of blocks. When, in order to avoid such a problem, the travel distance of the component block is reduced, an enormous amount of NC data on linear interpolation of sequential small moving blocks is required, the amount of arithmetic processing of the machining program increases, and a load on the controller for a motor for controlling the movement also increases. Thus, the machining time is increased.
Additionally, when there is an uncut, unmachined portion on the side opposite to the machined surface as shown in FIG. 13, the above-described machine tool has a disadvantage that, when the cutting tool is rotated and the cutting edge will cut this area if it has a large rotating radius. However, when a thin cutting tool with a cutting edge having a small rotating radius is used to avoid such a problem, an L/D (tool length/tool diameter) becomes large, and the rigidity of the tool degrades. As a result, there is a problem that the tool is bent, the tool chatters when machining, or the like.